History of infusoria, including the desmidiaceae and diatomaceae, British and foreign / by Andrew Pritchard.

  • Andrew Pritchard
Date:
1861
Catalogue details

Licence: Public Domain Mark

Credit: History of infusoria, including the desmidiaceae and diatomaceae, British and foreign / by Andrew Pritchard. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

    79/1074 (page 59)
    incmidual Hfc, lias been justly represented by Mr. Tliwaitcs as an act of ffemmation, not of reproduction. , , - „ In the coiu-se of seH-division, in some instances at least, a mucous or muco-gelatinous matter is thro^vn out around the frastules eng^g-ed. This circumstance did not escape the notice of Niigeh ; and Prof Smith {Synopsj^, i. p. 62) has, after noting it in previous pages as'a common phenomenon m the faroily, thus referred to it in the genus Pleurosigma :— While self-dl^aslon is actively going forward, the mucus generated by the di^dmg frustules is often so considerable as to produce the appearance and effect of a distinct fi-ond, which assumes the form of a thin peUicle of some little tenacity. At other times, when the mucous secretion does not assume the contmmty of a pellicle, it invests the individual ffustiile with a transparent envelope, which has the appearance of an exterior membrane, and has been sometimes mis- taken for such. On one occasion I edso met with, the frustules of P. Hi^Jpo- campiis enclosed in mucous or gelatinous tubes, precisely like those of a Colletonema; but these conditions must be regarded, for the present at least, as temporary or accidental, and cannot be admitted into the specific or generic descriptions. The process of self-division is affected in some unimportant particulars by the figui-e and habits of certain genera. Thus in one section of the Melosirece, the frustules of which have convex ends, Mr. Ralfs points out {A. N. H. xii. p. 347) that the central line is more strongly marked, and seems to divide the frustule iato two equal portions. It becomes broader, and at length double, and ultimately an intermediate growth separates the two halves of the frustule, which, during this process, do not increase in size ; but when the intermediate space is equal to the diameter of the original frustule, two new frustules are formed, by the addition of two hemispheres on the inner sides of the separated portions. The outer siUcious covering still remaining, the fmstules are connected in pairs, and appear like two globules within a joint, as they are characterized by Harvey in Melosira nummulaides, and by Carmichael in M. globifera. The above description belongs more particularly to M. nunimuloicles; but the process in the other species of this section is the same : a series of changes, nearly similar, occurs in Isthmia. In this genus, the author quoted says, the mode of growth is very curious. As in most of the Diatomeee, the plant increases by a division of the fmstiiles ; but in this genus, as also in Biddulphia and Anvphitetras (and in the Achnanthece), two new fi'ustules are formed within the old one, and as they enlarge, rupture it, when it falls off. In these the front portion is at first very narrow, and merely a broad line, but it increases greatly in breadth until the new frustules are fully formed. In this description and explanation the widening band or front portion mentioned is in fact the ' connecting membrane' of Prof. Smith, which, in the genera named, has an extra development, an extension beyond the sutures of the valves, and also an unusual persistence, retaining the two fnistules together after self- division, in such a manner that they seem to be enclosed mthin an origmal single fnistiilo, just as lb. Ealfs describes. This longer persistence of the connecting membrane has been noted by Prof. Smith (A. N. 11. 1851, p. 4), who wites— In some cases, by the new, or rather semi-new fnistules proceeding immediately to repeat tiic process [of self-division], the connecting membrane is thrown off and chsap- pears; in others it remains for some time, linldng tiic frustules iu pairs, as in Melosira and OfJontella. Another peculiarity, again, not unfi-equontly obtains in Ihis i)rocess of solf- fission, VIZ. a departure from the prevailing law of similarity which exists